postfix relay concern

[cuongvt]

From my dynamicIP (userA@xxx.dyndns.org) I already relayed ok through
my ISP (my email address provided from my ISP is ISPuser@ISP_smtp.com)
sending to outside my local LAN.
However, when I sent mail with "from" is userA@xxx.dyndns.org, to
outside mail address (ex: outsitesomewhere@mail.com),
userB who have mail address outsitesomewhere@mail.com noticed that
in the mail I sent him above, address in "from" is ISPuser@ISP_smtp.com, not
userA@xxx.dyndns.org. (I want it should show userA@xxx.dyndns.org though).
Could anyone help me how I can make it so that it will show userA@xxx.dyndns.org
in "from" address of userB?
Thanks
Below is my main config:
main.cf:
relayhost = [my isp smtp server]
smtp_generic_maps = hash:/etc/postfix/generic
transport_maps = hash:/etc/postfix/transport
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_mechanism_filter = plain, login
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = no
smtpd_sasl_security_options = noanonymous
smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,reject _unauth_destination

master.cf:
smtp inet n - n - - smtpd
smtps inet n - n - - smtpd
-o smtpd_sasl_auth_enable=yes
-o smtpd_client_restrictions=permit_sasl_authenticate d,reject

generic file:
userA@xxx.dyndns.org ISPuser@ISP_smtp.com
transport file:
my isp smtp server smtp:[my isp smtp server]

file sasl_passwd:
[my isp smtp server] ISPuser@ISP_smtp.com's usernameassword